RU2197542C1 - Method of making sheet steel - Google Patents

Abstract

FIELD: metallurgy; production of hot-rolled and cold-rolled steel sheets at high drawing properties for cold stamping. SUBSTANCE: steel containing the following components, mass-%: C, 0.002-0.007; Si, 0.005-0,050; Mn, 0.08-0.16; Al, 0.01-0.05; Ti 0,05-0.12; P≤, 0.015; S≤, 0.010; Cr≤, 0.04; Ni≤, 0.04; Cu≤, 004; N≤, 0/06 and Fe being the remainder is subjected to continuous teeming at rate of 0.4-1.6 m/min and temperature of 1500 to 1580 C in slabs which are heated to temperature of 1150-1240 C and are kept for 2.5-4 hours, after which they are rolled in strips at temperature of end of rolling process not below 870 C. Hot-rolled strips are cooled with water to temperature of 550 to 730 C and are rolled in rolls. After etching, strips are subjected to cold rolling at total cogging of no less than 70% and annealing at temperature of 700 to 750 C keeping them at this temperature for 11-34 h. Then, strips are subjected to skin-rolling at cogging of 0.4-1.2% at surface roughness of 2-4 mkm Ra and density of peaks of 60 to 120 l/cm. Provision is made for version according to which hot-rolled strips are subjected to annealing at temperature of 660-680 C followed by keeping them for 10-18 h. EFFECT: improved drawing properties; increased yield of sheet steel. 6 cl, 6 dwg, 1 ex

Description

 The invention relates to the field of metallurgy, and more particularly to a technology for manufacturing hot-rolled and cold-rolled steel sheets with high exhaust properties for cold stamping.

 Cold-rolled sheet steel with high exhaust properties for cold stamping of products of complex shape must meet a set of requirements for mechanical characteristics, their uniformity, microstructure, and surface roughness (Table 1).

 To ensure high exhaust properties, hot-rolled strips must satisfy the following requirements (Table 2).

A known method of manufacturing sheets for cold stamping, including continuous casting into slabs of steel of the following chemical composition, wt.%:
Carbon - not more than 0.008
Silicon - not more than 0.5
Manganese - no more than 1.0
Phosphorus - not more than 0.15
Sulfur - not more than 0.02
Aluminum - 0.01-0.10
Nitrogen - not more than 0.008
Titanium - 0.035-0.20
Niobium - 0.001-0.015
Iron - Else
The cast slabs are heated and subjected to rough hot rolling with a compression of at least 85% in the temperature range from Ar ₃ to 950 ^{o C.} , then the final hot rolling to a thickness of 3.5-7.5 mm in the temperature range from Ar ₃ to 600 ^o C. Hot-rolled strips are etched and annealed at a temperature of 700-920 ^o C. Then, the strip is cold rolled to a thickness of 1.2-1.6 mm with compression of at least 65% and annealed at a temperature of 720-920 ^o C [1].

 The disadvantage of this method is that it does not achieve high exhaust properties of cold-rolled sheets, resulting in their lack of stampability.

There is also known a method for the production of cold rolled sheet steel, deoxidized by aluminum. The method includes continuous casting into slabs of steel of the following chemical composition, wt.%:
Carbon - not more than 0.10
Manganese - not more than 0.60
Nitrogen - 0.0030-0.0100
Phosphorus - not more than 0.008
Sulfur - not more than 0.008
Iron - Else
The slabs are heated to a temperature of 950-1200 ^o C, rolled at a temperature above the point Ar ₃ and wound strips into rolls at a temperature not exceeding 600 ^o C. Hot rolled strips are etched and rolled on a cold rolling mill with a compression of 70-80%. Cold-rolled strips are heated at an average speed of not more than 100 ^o C / h to a temperature of 800 ^o C in bell-type furnaces and annealed. Annealed strips train [2].

 Sheet steel obtained by a known production method has a low coefficient of plastic anisotropy r = 1.42-2.01 (with the required r≥2.3) and the coefficient of strain hardening n = 0.18 (with the required n≥0.22) , unstable yield strength and unsatisfactory microstructural parameters (SCC score of more than 3). Therefore, sheet steel has insufficient exhaust properties with a low yield of conditioned steel.

The closest in its technical essence and the achieved results to the proposed invention is a method for the production of sheet steel for cold stamping (automobile sheet), including smelting and continuous casting in slabs of 08Y grade of the following chemical composition, wt.%:
Carbon - not more than 0.07
Manganese - 0.25-0.35
Silicon - 0.01
Phosphorus - not more than 0,020
Sulfur - no more than 0,025
Nickel - not more than 0.06
Copper - not more than 0.06
Chrome - no more than 0.03
Iron - Else
Continuously cast slabs are heated to a temperature of 1300 ^o C, rolled into strips with a temperature of the end of rolling 860-920 ^o C, cooled with water to a temperature of 550-650 ^o C and above, and then wound into rolls. Hot rolled strips are etched and cold rolled to the required thickness. Then the cold-rolled strips in rolls are annealed at a temperature of 680-690 ^o C for 30-40 hours and trained with compression of 1.0-1.5% [3] - prototype.

 The disadvantages of this method are that sheet steel due to low mechanical characteristics has insufficient exhaust properties, which leads to marriage during cold stamping of complex shapes. In addition, fluctuations in the chemical composition of steel and temperature-deformation conditions of production determine the variation in the parameters of mechanical properties both along the length of the strips and from melting to melting. As a result, the yield of conditioned sheet steel is reduced.

 The technical problem solved by the invention is to improve the exhaust properties and increase the yield of conditioned sheet steel.

The technical problem is solved in that in the known method for the production of sheet steel for cold stamping, including continuous casting of slabs, heating and hot rolling into strips, etching, cold rolling, annealing and tempering, according to the invention, the slabs are heated to a temperature of 1150-1240 ^o C is rolled with the rolling finishing temperature not lower than 870 ^o C, cooled with water to a temperature of 550-730 ^o C and wound into rolls, and cold rolling the lead with a total rolling reduction of not less than 70%, then annealed at a temperature of 700-750 ^o C with extracts at this temperature for 11-34 hours.

In embodiments of the method, continuous casting is conducted at a speed of 0.4-1.6 m / min at a temperature of cast steel 1500-1580 ^o With, and slabs are cast from steel containing, wt.%:
Carbon - 0.002-0.007
Silicon - 0.005-0.050
Manganese - 0.08-0.16
Aluminum - 0.01-0.05
Titanium - 0.05-0.12
Phosphorus - not more than 0.015
Sulfur - not more than 0.010
Chrome - no more than 0.04
Nickel - not more than 0.04
Copper - not more than 0.04
Nitrogen - no more than 0,006
Iron - Else
In addition, before hot rolling, the slabs are kept at a heating temperature for 2.5-4 hours, and cold-rolled annealed strips are trained with 0.4-1.2% compression in rolls with a surface roughness of 2-4 μm Ra and a peak density of 60 -120 1 / cm.

A possible embodiment of the method in which the hot-rolled strip is subjected to annealing at a temperature of 660-680 ^o With exposure at this temperature for 10-18 hours

The invention consists in the following. High exhaust properties of sheet steel for cold stamping of complex products are laid at the stages of smelting and casting with the limitation of alloying elements and impurities. The proposed composition of ultra-low-carbon steel makes it possible to form a uniform ferritic microstructure during its further thermoplastic processing, in which the excess carbon is bound by titanium, nitrogen forms aluminum and titanium nitrides, which excludes steel aging, accompanied by degradation of mechanical properties. In the process of continuous casting of such steel at a temperature of 1500-1580 ^o With a speed of 0.4-1.6 m / min, an ingot is formed with minimal segregation and the absence of segregation of non-metallic inclusions in the axial part of the ingot.

In the process of heating slabs before rolling to a temperature of 1150-1240 ^o C and holding for 2.5-4 hours, large carbosulphides are not dissolved in austenite, as a result of which, after winding a hot-rolled strip into a roll, as well as during annealing of the cold-rolled strip, the formation of small carbides inhibiting the growth of ferrite grains. Subsequent hot rolling of slabs in the austenitic region with a temperature of the end of rolling not lower than 870 ^° C, in addition to obtaining a predetermined strip thickness before cold rolling, forms a favorable deformation texture and crushes austenite grains. Hot rolling in a given temperature range ensures the precipitation of TiC, TiS, TiN ₂ dispersed particles during deformation, which remain in the recrystallized structure after annealing. The cooling of hot rolled strips with water to a temperature of 550-730 ^o C (below the critical point Ar ₁ ) leads to the transformation of deformed grains of supercooled austenite into ferrite with small equiaxed grains and stabilization of carbon when it is released in the form of carbides and carbosulfides. The microstructure of steel formed after hot rolling is characterized by optimal dispersion and a high degree of homogeneity.

During the subsequent cold rolling of hot-rolled strips with compression of at least 70%, the specified thickness of the strips and the degree of grinding of the deformed ferritic grains are achieved. As a result of annealing of cold-rolled strips at a temperature of 660-680 ^o C for 10-18 hours, a uniform microstructure of ferrite with a grain score of 6-7 and a minimum release of structurally free cementite is formed. Particles of TiC, TiN ₂ have an inhibitory effect on the recrystallization of grains during annealing. After annealing, the crystallographic orientation (111), which is most favorable for cold stamping, prevails in the steel texture.

Training of annealed strips with a compression of 0.4-1.2% in rolls with a roughness of 2-4 μm Ra and a peak density of 60-120 1 / cm, firstly, allows to increase ductility, reduce the limit by 30-40 N / mm ² fluidity, eliminate the area and tooth yield on the tensile diagram of the sample (to avoid slip lines during stamping), and secondly, to create developed microgeometry that holds the technological lubricant on the sheet surface, which improves the formability of complex products.

Annealing of hot-rolled pickled strips at a temperature of 660-680 ^o C with a holding time of 10-18 h provides a given set of mechanical properties (Table 2) even with an unfavorable combination of the concentration of chemical elements and inevitable fluctuations in the temperature-deformation modes of rolling. Due to this annealing, the exhaust properties are further improved and the yield of conditioned steel sheet is increased. Annealed hot-rolled strips can be used both for stamping products where high exhaust properties are not required, and for the subsequent production of cold-rolled sheet steel with increased stampability.

It was experimentally established that heating slabs to a temperature above 1240 ^o C leads to an increase in austenite grain, the dissolution of large particles of carbosulfides, which subsequently precipitate in the form of small carbides, reinforcing steel, impairing the elongation properties of the sheets. Lowering the heating temperature below 1150 ^o C degrades the plastic properties and stampability of the sheets.

When the temperature of the end of rolling is lower than 870 ^o С, the sheet steel acquires a fine-grained structure and a texture unfavorable for cold stamping.

An increase in the winding temperature above 730 ^o C leads to heterogeneity of the microstructure of the bands, a decrease in the coefficient of plastic anisotropy. In this case, hot rolled strips even after annealing do not meet the requirements for mechanical properties. A decrease in winding temperature below 550 ^o C leads to an increase in the strength properties of hot-rolled and cold-rolled sheets above acceptable values.

 When the total compression during cold rolling is less than 70%, sheet steel after annealing has an unfavorable texture, which degrades the exhaust properties and reduces the yield of conditioned products.

When the annealing temperature of cold-rolled strips is lower than 700 ^o C or the exposure time is less than 11 hours, the ductility of the sheet steel is lower than the permissible one, the CCC score reaches 3. An increase in the annealing temperature of more than 750 ^o C or the exposure time of more than 34 hours leads to an increase in the different grain size, deterioration of formability and a decrease in the yield of conditional sheet steel.

Continuous casting of steel having a temperature below 1500 ^o With a casting speed of less than 0.4 m / min, leads to the formation of an ingot with heterogeneous chemical composition, non-metallic inclusions and impurities are concentrated in the middle part of the ingot, the mechanical properties of sheet steel are deteriorating. An increase in casting temperature of more than 1580 ^o C or casting speed of more than 1.6 m / min affects the quality of the sheets and reduces the yield of conditioned products.

 Carbon in steel is a reinforcing element. A decrease in carbon content of less than 0.002% complicates the process and increases the cost of steel production. An increase in carbon concentration in excess of 0.007% requires an increase in the content of titanium, which binds carbon. This affects the properties of the sheets and increases the consumption of alloying.

 Silicon deoxidizes steel. A decrease in the silicon content of less than 0.005% increases the oxidation of steel, its mechanical properties deteriorate. An increase in the content of this element over 0.050% leads to a loss of ductility, an increase in the number of non-metallic inclusions.

 Manganese has a strengthening, deoxidizing and desulfurizing effect. When the manganese content is less than 0.08%, additional desulfurization of steel is required, which increases the cost of production, and an increase in its content of more than 0.16% affects the ductility and viscosity properties, reduces the coefficient of plastic anisotropy.

 Aluminum is introduced to deoxidize steel and reduce casting defects. With a decrease in aluminum content of less than 0.01%, steel becomes prone to degradation of mechanical properties (aging). An increase in aluminum content of more than 0.05% leads to a deterioration in its mechanical properties below an acceptable level.

 Titanium is introduced to improve the complex of mechanical properties of steel. When the titanium content is less than 0.05%, deep decarburization is required, which complicates and increases the cost of production. An increase in titanium content of more than 0.12% leads to a decrease in the coefficients of plastic anisotropy and strain hardening below an acceptable level.

 Sulfur and phosphorus are harmful impurities that impair the mechanical properties of hot-rolled and cold-rolled sheets. However, when the phosphorus content is not more than 0.015% and sulfur is not more than 0.010%, their harmful effect is weak. At the same time, deeper desulfurization and dephosphorization significantly increase the cost of steel.

Chrome, nickel and copper strengthen steel, but at a concentration of more than 0.04% of each, there is a loss of ductility, and the ratio of σ _t / σ _in exceeds 0.76, which is unacceptable.

 Nitrogen strengthens the steel, but if its amount exceeds 0.006%, the amount of titanium required for its bonding increases, which makes its production more expensive and affects the properties of cold-rolled sheets, reduces the yield of conditioned sheet steel.

 In the case of holding the slabs at a heating temperature of less than 2.5 hours, the temperature field is not equalized in thickness and the austenitic state is completely stabilized, and an increase in holding time in excess of 4 hours leads to an excessive growth of austenitic grain and complete dissolution of carbosulfides. As a result, the exhaust properties of cold-rolled sheets deteriorate, and the yield decreases.

 Training with compression of less than 0.4% in rolls with parameters of surface roughness Ra less than 2 μm and Pc less than 60 1 / cm does not reduce the yield strength, eliminate the tooth and yield area, and create developed microgeometry of the surface of cold-rolled sheets. As a result, their exhaust properties deteriorate and the yield of conditioned sheet steel decreases. The increase in compression is more than 1.2%, the parameters Ra more than 4 μm and Pc more than 120 1 / cm leads to re-hardening of the cold-rolled annealed strips and the creation of anisotropy of the surface roughness due to an increase in the contact slip of the metal over the roll surface in the deformation zone. As a result, the exhaust properties of sheet steel are deteriorated.

Annealing of hot-rolled strips at a temperature below 660 ^o C or a holding time of less than 10 hours does not provide high exhaust properties that are uniform along the length of the roll. An increase in the annealing temperature of more than 680 ^° C or a holding time of more than 18 hours leads to the formation of a mixed-point microstructure, deterioration of the exhaust properties, and a decrease in the yield of conditioned steel sheet.

An example implementation of the method
In an oxygen converter with a capacity of 300 tons smelted ultra-low carbon steel of the following composition, wt.%
C - 0.0045
Si - 0,028
Mn - 0.13
Al - 0.03
Ti - 0.09
P - 0.010
S - 0.007
Cr - 0.01
Ni - 0.01
Cu - 0.02
N - 0.003
Fe - Else
The smelted steel is cast on a continuous casting machine into slabs with a section of 250x1280 mm and a mass of 11 tons. Casting is carried out at a speed of V _p = 1.0 m / min at a temperature of the cast metal T _p = 1540 ^o C.

After cooling, the molded slabs are loaded into a gas furnace with walking beams, heated to an austenitizing temperature T _a = 1200 ^° C, at which they are kept for a time τ _a = 3.2 hours. The slabs are successively pushed onto a continuous rolling mill 2000 continuous rolling table and pressed into roll section of 40x1300 mm. Then the roll set in a continuous 7-stand group and rolled to a final thickness of 2.8 mm The temperature of the strips at the exit from the last stand of the mill finishing group is maintained equal to T _kn = 880 ^° C. The hot-rolled strips on the discharge roller table are cooled with water to a temperature of T _cm = 640 ^° C and wound into rolls.

 Chilled rolls are subjected to sulfuric acid pickling in a continuous pickling unit.

Холоднокатаные полосы в рулонах загружают в садочную печь с водородной защитной атмосферой и отжигают при температуре Т_о=730^oС с выдержкой в течение τ_o = 22 ч. Отожженные полосы дрессируют на одноклетевом стане кварто с текстурированными рабочими валками, шероховатость которых равна Ra=3 мкм; Рс=90 1/см. Дрессировку ведут с обжатием ε_д = 0,8%.Then the etched strips in rolls are rolled on a 5-stand mill of quarto endless cold rolling from a thickness of 2.8 to 0.7 mm with a total compression ε _Σ equal to

Cold rolled strips in rolls are loaded into a hydrogen furnace with a hydrogen protective atmosphere and annealed at a temperature of Т _о = 730 ^o С with holding for τ _o = 22 h. The annealed strips are trained on a single-unit quarto mill with textured work rolls, the roughness of which is Ra = 3 microns; Pc = 90 1 / cm. Training is carried out with compression ε _d = 0.8%.

 Table 3 gives the chemical compositions of the proposed and known steel, in table. 4 - modes of production of cold rolled sheets, and in table 5 - exhaust properties and the output of the conditioned sheet.

 From the table. 3-5, it follows that in the case of the implementation of the proposed method (options 2-4), an improvement in the exhaust properties and an increase in the yield of conditioned steel sheet are achieved. With exorbitant values of the declared parameters (options 1 and 5) and the use of the prototype method (option 6), the exhaust properties of the sheets and the output of the conditioned steel sheet are reduced.

Part of the hot rolled pickled rolls is sent to recrystallization annealing. Annealing is carried out in a protective atmosphere of nitrogen according to the regime: heating to a temperature of T _og = 670 ^o C, holding for τ _og = 14 h, cooling under a muffle.

Annealing options and properties of hot-rolled annealed sheet steel are given in table. 6. Annealing of hot-rolled strips at a temperature of 660-680 ^o C for 10-18 hours (options 2-4) provides a complex of mechanical properties that fully comply with the required properties given in table. 2.

 As a basic object in determining the technical and economic advantages of the proposed method adopted the prototype method. Using the proposed method will increase the profitability of the production of sheet steel with high exhaust properties for cold stamping by 10-15%.

Literature
1. Application 0936279 (EPO), IPC C 22 C 38/00, 1999

 2. Application 59-13030 (Japan), IPC C 21 D 9/48, C 21 D 8/04, 1984

 3. S. S. Guseva and others. Continuous heat treatment of steel sheet. - M.: Metallurgy, 1979, p. 9-26 is a prototype.

Claims (5)

1. A method of manufacturing sheet steel for cold stamping, including continuous casting of slabs, heating and hot rolling into strips, water cooling, winding into coils, pickling, cold rolling, annealing and tempering, characterized in that the slabs are heated to 1150-1240 ^o C, rolled with a temperature of rolling end not lower than 870 ^o C, cooled with water to 550-730 ^o C and wound into rolls, and cold rolling is carried out with a total compression of at least 70%, then annealed at 700-750 ^o C with holding at this temperature for 11-34 hours 2. The method according to p. 1, characterized in that the continuous casting of slabs is carried out at a speed of 0.4-1.6 m / min at a temperature of the cast steel 1500-1580 ^o C. 3. The method according to PP. 1 and 2, characterized in that the slabs are cast from steel containing, by weight. %:
Carbon - 0.002-0.007
Silicon - 0.005-0.050
Manganese - 0.08-0.16
Aluminum - 0.01-0.05
Titanium - 0.05-0.12
Phosphorus - Not more than 0.015
Sulfur - Not more than 0.010
Chrome - Not more than 0.04
Nickel - Not more than 0.04
Copper - Not more than 0.04
Nitrogen - Not more than 0.006
Iron - Else
4. The method according to p. 1, characterized in that the slabs are kept at a heating temperature for 2.5-4 hours  5. The method according to p. 1, characterized in that the training of the bands is carried out with compression of 0.4-1.2% in rolls with a surface roughness of 2-4 μm Ra and a peak density of 60-120 1 / cm 6. The method according to p. 1, characterized in that the hot-rolled strip is annealed at 660-680 ^o With exposure at this temperature for 10-18 hours

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